In some prior art tool bodies, strain gauges are installed for getting the twisting forces applied to the tool body. However generally, theses strain gauges have no sufficient sensitivities for providing a precise twisting force value. Thereby it will cause that the tool body with precise control cannot be used.
In one improvement, the tool body is formed with a round recess and the strain gauge is installed in the recess. However since the recess has a round shape and the strain gauge is installed at a center of the recess, but is operation, the stresses of the tool body are concentrated on the lateral wall of the recess and thus as a result, the deformation of the strain gauge is smaller and thus it cannot provide a precise value about the measured twisting force. Thereby this way can be effectively increase the sensitivity of the strain gauge.
In another improvement, U.S. Pat. No. 4,006,629, two sides of a handle tool are formed with grooves and four strain gauges are installed in the grooves. However in the patent, a conductive wire for a display of the handle tool is exposed out. It is easy to be collided or broken so as to make the strain gauges cannot work. Thereby the structure is complicated and four strain gauges are used. These all increase the cost of the handle tool.